It is well-known in the art that by the measurement of the capacitance change versus time curves of the depletion region or space charge layer near PN junctions in semiconductor devices the concentration, activation energy, thermal and optical capture cross-sections of localized states (created by foreign atoms, point defects, dislocations, etc.) in the forbidden gap of the semiconductor devices can be determined.
The change of the occupancy of the localized states in the forbidden gap can be measured as a change of the capacitance. During such measurements the localized states are periodically filled and emptied, and the amplitude of the resulting periodical capacitance-change is determined by the concentration of the energy states, while the kinetics thereof is determined by the activation energies and thermal capture cross-sections of the localized states as a function of temperature.
The occupancy of the localized states are changed by voltage-, current- or light-pulses applied to the PN junction. This measuring method is the deep level transient spectroscopy or DLTS method as referred to hereinabove, and the instruments utilising this method are indispensable factors of development and quality control works in the field of semiconductor devices.
The DLTS method is described in the paper of G. L. Miller, D. V. Lang and L. C. Kimerling entitled "Capacitance Transient Spectroscopy" (Ann. Rev. Metr Sci. 1977, pp 377-348) and here the authors also give a survey on the instrumentation used for the measurements of transient capacitance changes.
The measurement of the capacitance changes is carried out in such a way that the diode representing the semiconductor junction to be measured is connected into one arm of a bridge, and a compensating capacitance is inserted in the other arm of the bridge having a capacitance equal to that of the diode. A high frequency signal is applied to the two arms of the bridge through a symmetrical high frequency transformer.
In addition to the application of the high frequency signal a train of pulses is applied to the diode through a pulse-transformer. The repetition rate and the duty cycle of the pulses can be adjusted within wide ranges in accordance with the DLTS method. During the occurrence of the pulses a forward or slightly reverse voltage is applied to the diode, and between the ending moments of the pulses and the starting moments of the subsequent pulses a high reverse voltage is applied to the diode. The capacitance of the diode is substantially different under the two kinds of voltages applied thereto which difference can be as high as 2-3 decimal orders of magnitude, and during the presence of the reverse bias a capacitance transient (a decay) takes place which is typically by 4 decimal orders of magnitude smaller than the d.c. reverse capacitance.
The task of the measurement is to determine the small capacitance changes during the reverse biasing pulses.
In a known measuring apparatus the output of the measuring circuit is rectified by a phase-sensitive (lock-in) rectifier following a sufficient amplification, and the so obtained pulsating rectified voltage will be proportional to the capacitive output signal component of the measuring circuit. This voltage is applied to the input of a low-frequency further lock-in amplifier synchronized to the starting moments of the measuring periods, and the second lock-in amplifier selects the component from the rectified pulsating input voltage which has the same frequency as the measuring periods have.
During performance of the DLTS method the temperature and other parameters of the diode to be measured are changed and such changes take place much slower than the rate of pulses applied to the diode. The information characterizing the properties of the diode is carried by the registrated sequence of capacitance changes obtained during the pulse periods with reverse bias.
There are a number of problems connected with the known apparatus summarized above.
The application of the excitation pulses is rather complicated, and in the described arrangement the stray capacitance and series inductance of the pulse transformer can result in pulse parameters other than the set values, especially if there are high conductive losses. The true transmission of pulses with durations that can be varied within a wide range, which wide range is required for the measurements of the capture cross-section of majority or minority carriers, can be solved precisely by using a plurality of pulse-transformers only.
The measuring amplifier receives the full driving signal, and following the significant overloading it can recover relatively slowly, i.e. the ineffective death period of the capacitance measurement will be long, i.e. the starting section of the exponentially decaying capacitance signal will be lost for the measurement, which all result in a substantially decreased signal to noise ratio compared to an ideal case. If the starting of the reference signal of the low-frequency lock-in amplifier is phase-locked to the starting or ending moments of the biasing pulses, then the death periods will have different lengths when the width and repetition rate of the pulses are changed, which result in inaccurate determinations of activation energies (D. S. Day et al. Journal of Applied Physics, 50/8, 5093/1979).
High sensitivity can be reached by averaging throughout a long period of time defined generally by the integrating time-constant of the low frequency lock-in amplifier. If the high-frequency gain is high, the high-frequency phase-sensitive rectifier, which can have only short time constant for the sake of good transient response, might overdrive the low frequency lock-in amplifier that has a long time constant and a corresponding long recovery period following an overdriving. In order to overcome such unwanted side-effects, neither the high-frequency, nor the low frequency gain can be sufficiently high, i.e. the sensitivity of the capacitance measurements cannot be increased as it would be desirable.